Substrates on which an electronic device are electrically mounted are commonly fabricated from dielectric materials such as ceramics, ceramic metal composites, polymeric materials and polymeric material metal composites. These substrates can have multi-level electrical conductor patterns imbedded therein.
In a simplest embodiments the substrate has one dielectric layer having electrical conductor patterns on both sides thereof which are electrically interconnected by a plurality of electrically conducting vias extending through the substrate.
In another embodiment, the substrate is fabricated from an electrically and/or thermally conducting layer coated with a dielectric material. A plurality of through-holes are drilled, punch or etched through the substrate. To form an electrically conducting via which is electrically insulated from the core requires that the through-hole sidewall be coated at least in the region of the core, with an electrically insulating material.
One method of coating the sidewall of the through-hole with a dielectric material is to dip the substrate into a liquid polymer. For the liquid polymer to get into the through-hole requires that the liquid polymer have a sufficiently low viscosity. Generally, to achieve such a low viscosity requires that the liquid polymer be mixed with a solvent. If it is desired that only the sidewall of the through-hole be coated with the dielectric material, the liquid polymer must have a high solvent content. Moreover, to promote wetting of the sidewall by the dielectric coating requires that the liquid polymer have a high solvent content. The substrate must then be cured or heated to volatize the solvent contained in the liquid polymer. Where the liquid polymer has a high solvent content, securing of the liquid polymer results in a substantial shrinkage of the thickness of the liquid polymer layer coating the through-hole sidewall. Generally, to achieve a polymeric sidewall thickness which is sufficiently thick to reliably avoid electric shorting between the core and the electrically conductive via which will be deposited in the sidewall coated through-hole, requires multiple immersions in the liquid polymer and multiple curing steps of build up the sidewall thickness.
A method according to present invention avoids the use of liquid polymers containing solvents to coat the sidewalls of through-holes and also avoids the multiple step process to achieve adequate sidewall dielectric thickness.
Another method of coating the sidewall of a through-hole in a substrate containing an electrically conducting core is electrophoretic deposition. In electrophoretic deposition the substrate with through-holes therein is immersed in a solution containing polymer particles. An electrode is immersed in the solution and a potential is applied between this electrode and the electrically conductive core of the substrate. Particles in the solution enter into the through-holes and there is an electronic exchange at the exposed electrically conductive core in the sidewall of the through-hole which results in the deposition of the polymeric material thereon. This results in a relatively high current at the electrically conductive core part of the sidewall of the through hole. If this current is too high there is a rapid generation of gas which prevents effective electrophoretic deposition. If the current is too low there is no electrophoretic deposition. Therefore, electrophoretic deposition requires precise control of the current to effectively deposit the polymeric sidewall.
According to the present invention the disadvantages of the above described techniques are avoided. According to one aspect of the present invention, the dielectric material is heated and then pressed. Quite surprisingly it has been found that the dielectric material which gets pressed into the through-holes can be controlled to coat only the sidewall of the through-hole or to completely fill the through-hole without trapping therein voids which would result in defects in the dielectric material either coating the sidewall of the through-hole or completely filling it.
Quite surprisingly it has been found that a relatively large number of through-holes on a relatively large substrate can be coated with a dielectric material by the method according to the present invention. Where a through-hole has been completely filled by the method according to the present invention a through-hole through the enter of the dielectric material filling the substrate through-hole can be thereafter formed by drilling, punching or etching.
It is an object of this invention to either coat the sidewall of a hole in a substrate or to completely fill the hole in a substrate with a material by pressing the material without trapping voids in the dielectric material.